The current development towards truly mobile computing and networking has brought on the evolvement of various access technologies that also provide the users with access to the Internet when they are outside their own home network. At present, wireless Internet access is typically based on either short-range wireless systems or mobile networks, or both.
Short-range wireless systems have a typical range of one hundred meters or less. They often combine with systems wired to the Internet to provide communication over long distances. The category of short-range wireless systems includes wireless personal area networks (PANs) and wireless local area networks (WLANs). They have the common feature of operating in unlicensed portions of the radio spectrum, usually either in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band or in the 5 GHz unlicensed band.
Wireless personal area networks use low cost, low power wireless devices that have a typical range of about ten meters. The best-known example of wireless personal area network technology is Bluetooth, which uses the 2.4 GHz ISM band. It provides a peak air link speed of one Mbps, and power consumption low enough for use in personal, portable electronics such as PDAs and mobile phones. Wireless local area networks generally operate at higher peak speeds of 10 to 100 Mbps and have a longer range, which requires greater power consumption.
Wireless LAN systems are typically extensions of a wired network, providing mobile users with wireless access to the wired network. Examples of wireless local area network technology include the IEEE 802.11a, which is designed for the 5 GHz unlicensed band, and uses orthogonal frequency division multiplexing (OFDM) to deliver up to 54 Mbps data rates; the 802.11b, which is designed for the 2.4 GHz ISM band and uses direct sequence spread spectrum (DSSS) to deliver up to 11 Mbps data rates; and the HIPERLAN Standard, which is designed to operate in the 5 GHz unlicensed band.
In wireless LAN technology, two basic network topologies are available for network configuration: an ad-hoc network and an infrastructure network. An ad-hoc network is formed by two or more independent mobile terminals without the services of a base station, i.e. in an ad-hoc network the terminals communicate on a peer-to-peer basis. An ad-hoc network is normally formed for temporary purposes. The infrastructure network, in turn, comprises one or more wireless base stations, called access points, which form part of the wired infrastructure. In a typical network of this type, all traffic goes through the access points, regardless of whether the traffic is between two terminals or a terminal and the wired network, i.e. the mobile terminals do not communicate on a peer-to-peer basis. The mobile terminals are provided with wireless LAN cards, whereby they can access the wired network or set up an ad-hoc network. In an infrastructure network an access point and at least one terminal is said to form a Basic Serving Set (BSS), while an ad-hoc network is also termed an Independent BSS (IBSS).
So far, WLAN technology has been used mainly in laptop computers, which are typically AC powered but which may also be used in battery mode that provides a fairly high battery capacity. To prolong the life of the batteries, the WLAN standards define a specific power save mode into which the terminals may enter in order to decrease their power consumption. In this mode the power consumption is very low, but the terminals have to wake up periodically to receive regular beacon transmissions broadcast in the network. In a BSS, the beacon transmissions indicate, for example, whether there are incoming packets buffered for a terminal. If so, the terminal retrieves the packets, goes back to sleep, and wakes up again to listen to the next beacon transmission.
In an ad-hoc network (IBSS), where no access points exist, one of the wireless terminals assumes the responsibility of sending the beacon frame. The beacon interval of the ad-hoc network is set by the terminal that instantiates the ad-hoc network. This terminal initiates a series of Target Beacon Transmission Times (TBTTs). At each TBTT, each terminal calculates a random time delay and then broadcasts a beacon frame if no other terminal does so before that. The purpose of the random time delay is to circulate the beacon broadcast responsibility among the terminals of the ad-hoc network.
After each TBTT, an ATIM (Announcement Traffic announcement message) window follows, during which only beacon frames and ATIM frames can be sent. ATIM frames are sent by terminals that have buffered frames for the terminals that are in power save mode, and an ATIM frame thus indicates to a terminal that buffered data waits to be delivered to the said terminal. An ATIM frame is either sent directly to one terminal or multicast/broadcast to several terminals. A dedicated ATIM frame has to be acknowledged by the receiving terminal. All the terminals have to be awake during the ATIM window, and if a terminal receives an ATIM frame, it has to be awake for the next beacon interval. A terminal may receive multiple ATIM frames from several other terminals, and it has to acknowledge each dedicated ATIM frame received. If a terminal has buffered data for several other terminals, it has to send a dedicated ATIM frame to each of said terminals. For sending each ATIM frame or acknowledgment, the terminal has to contend with other terminals for access to the channel.
In an ad-hoc network, the terminal that instantiates the network decides on whether power saving is used in the network. If the power saving is not used, the value of the ATIM window is set to zero.
A drawback that relates to the above-described operation of an IBSS is that the proportion of the time during which the terminals have to be awake is relatively high. In other words, the total time that the terminals of an ad-hoc network have to be awake just as a precaution in case they should have to perform some transmission/reception functions is rather high. Furthermore, it is inefficient that in case of several terminals having buffered data for the same terminal, several ATIM frames have to be sent. This all causes a lot of overhead in terms of power consumption and shortens the life of the terminal batteries.
The present invention seeks to accomplish a solution by means of which the above drawbacks in the operation of an IBSS can be alleviated.